This invention is concerned with improving the protective ability of a coating having a higher thermal expansion coefficient than the metal it covers. The invention is particularly directed to protecting space shuttle reentry thermal protection systems, aircraft gas turbine engine components, and other applications where coatings must provide environmental protection for refractory metals. Superalloys can also be protected in this manner.
Conventional slurry silicide coatings have been modified to change the composition of the external metal di-silicide or the sub-silicide layers which form between the di-silicide and the refractory metal to be protected. Such modifications have, in essence, changed the oxides which form upon silicide coating oxidation. Extra oxides form in addition to silicon dioxide which is the primary protective oxide. These secondary oxides are readily formed at intermediate temperatures thereby enabling the silicides to resist catastrophic intermediate temperature oxidation, commonly called silicide pest. They also aid in filling the cracks formed in most silicide coatings and thus block direct exposure of the crack tips to air.
Cracks are formed in silicide coatings because of the large thermal expansion mismatch between metal silicides and the refractory metals. More particularly, the thermal expansion coefficient of the metal silicides is generally greater than that of refractory metals. Since silicide coatings generally have columnar grain structures oriented perpendicular to the substrate, the crack formations at the grain boundaries produce short paths between the oxidizing environment and the refractory metal substrate. Such cracks are potential oxidation failure sites.